It is known that exhaust gasses from a compression-engine, such as a diesel engine, typically contains NOx, which consists primarily of nitric oxide (NO) and approximately 5 to 20 percent NO2. Production of NOx may be particularly problematic when operating lean of stoichiometric as a result of the relatively high levels of oxygen in the exhaust gas stream. To address this issue, an exhaust after-treatment system may include a lean nitrogen-oxides trap (LNT) where a NOx adsorbent material, such as a zeolite, is disposed.
Once the NOx adsorbent material in an LNT is saturated, the effectiveness of the LNT decreases substantially, and NOx may slip past the LNT. For this reason, it can be particularly important for an LNT to have sufficient adsorption capacity available. The efficiency of an LNT may be maintained either by replacing the component or by periodic cleaning or regeneration, but in order to avoid service interruptions, regeneration, and in particular a denitrification (DeNOx) regeneration event is generally preferred.
A DeNOx regeneration event may be operated by switching the engine from a conventional lean-combustion mode to a rich-combustion mode. When the engine is switched to the rich-combustion mode, the NOx stored on adsorbent sites of the LNT reacts with reductants contained in the exhaust gas, such as unburned Hydrocarbons (HC), and are desorbed and converted to nitrogen (N2) and ammonia (NH3).
For the purpose of guaranteeing safe (DeNOx) regeneration events, generally, the electronic control unit (ECU) of the internal combustion engine is configured to activate the regeneration events only if the internal combustion engine is operated according to engine operating points confined into a pre-calibrated area, i.e. a so-called rich mode area, of the engine speed-engine torque diagram of the internal combustion engine.
Since this rich purge strategy penalizes the fuel consumption, a need exists for a method that minimizes the fuel penalty optimizing the LNT conversion efficiency and the emission benefits.